1. Field of the Invention
The invention is in the field of sputtering targets.
2. Description of the Related Art
Sputtering is a manufacturing process that achieves thin film deposition by bombarding a target material with energetic ions. The sputtering target material becomes deposited on a work piece. Some of the products made by sputtering include mirrors, compact discs, low-E insulating glass, architectural glass, and flat panel displays.
There are two main types of sputtering targets, including planar and rotary types, each having its own set of advantages and limitations dependent upon the coating application. In large area coatings, such as architectural glass, high rates of speed and long life of the targets are desired to lower manufacturing costs and reduce downtime of the systems. In this example, rotary targets have many advantages over planar technologies and are finding wide acceptance in the industry. However, rotary targets are more complex and can be very difficult to make depending on the desired materials to be sputtered and the coating application. Because of the complexity and cost to manufacture rotary targets out of certain sputter materials, some materials are still sputtered or deposited using other techniques and cannot economically or technically take advantage of the rotary target sputter process. It is possible to sputter many different materials including for example metals, metal alloys, ceramics, nitrides, and oxides. This is just a representation of materials and not an inclusive list of application materials. Some target materials are very expensive and are more easily handled in the manufacturing process in smaller sizes until final assembly of the target. Others can be melted and poured into a mold around a backing tube and machined to final specifications after the pour. Some target materials can be very brittle, such as some materials like ITO (indium-tin oxide) used in the flat panel display industry. To construct long cylinders of the target material to be fit into a rotary cathode can be cost prohibitive with traditional techniques known in the art. In addition, some target materials are easily destroyed during manufacturing due to a variety of reasons including but not limited to brittleness, thermal sensitivity, low impact strength, bonding failures, differing rates of thermal expansion and other properties. In the sputtering process, cycling temperatures, vacuum conditions, high sputter surface plasma temperatures, fixturing integrity, liquid cooling of the tube, operating long term at high power levels, and other parameters can all contribute to the failure or pre-mature failure of the rotary target.
The typical way of making rotary sputtering targets using brittle materials for example, has been to bond the brittle target material to a stainless steel backing tube with an indium-based bonding material. Indium as a bonding material has a relatively low melting temperature of 157 degrees C (314 degrees F). Because of the low melting temperature, it can withstand only a low amount of thermal stress. Indium-based bonding material is very expensive, costing around U.S. $1102 per kilogram (U.S. $500 per pound) at the time this specification is being written. However, the low melting temperature of the bonding material is also a disadvantage under certain conditions, such as a coolant failure or flow restriction of the cooling system that maintains an adequate process temperature of the cathode during its use. A system cooling failure may inadvertently raise the target temperature above the bonding material's melting point resulting in a failure of the target and significant expense, and/or downtime of the system involved. As an additional example, a process may be desired that would operate the target at temperatures that may exceed the melting point of the bonding material, perhaps toward the end of the target's rated life where the erosion pattern of the target gets closer to the bonding layer of the target. A target manufactured with a higher melting point bonding material would be less sensitive to the process application and be able to operate at higher power levels and faster speeds utilizing bonding materials appropriate to the application in terms of performance and cost. The ability to use a broader range of bonding materials allows the optimization of the rotary target design to accommodate different target material's thermal, mechanical, and processability requirements. Bonding materials appropriately selected for a specific process can improve performance, reduce cost, and improve reliability. In a coating system for architectural glass for example with a large number of targets installed, these advantages become very significant in terms of reduced operating cost, less downtime, and faster process speeds.